Notify organizer when the last activity in TaskFragment enter PiP

When the Task contains multiple activities, activity entering PiP will
be reparented to a new PiP Task. In this case, we should notify the
organizer about the behavior, and prevent the dependents from closing as
a result of an empty TaskFragment.

Bug: 225371112
Test: atest WmTests:TaskFragmentTest
Change-Id: I6f0138c0532d2d0249de38fd84d1a4690f266c07